Small hollow glass spheres (microspheres) are useful as fillers in various organic and inorganic matrices for a wide variety of industrial applications. These microspheres can be used to achieve special properties such as low dielectric constant, high strength-to-weight ratio, and low thermal conductivity.
In electronics applications, microspheres having low sodium content are often preferred. Sodium cations are highly mobile positive charge carriers which can have adverse effects on the performance of electronics exposed to the cations.
While low sodium content in the glass microspheres is desirable for electronics applications, low sodium glasses present difficulties in the manufacture of microspheres. Low sodium glasses generally have higher viscosities at higher temperatures than conventional glass compositions. Blowing or expansion agents, used to promote expansion of the glass into hollow spheres, are often volatilized before the low sodium glass reaches a low enough viscosity for microsphere formation.
Conventional methods for producing low sodium glass microspheres from aqueous precursors involve production of microspheres from relatively high sodium content glass. An aqueous sodium silicate precursor (i.e. water glass) is formed into droplets and heated to form high sodium microspheres which are then treated to remove the sodium by leaching or some other method. This sodium removal step adds extra time and cost to the production of low sodium microspheres.
Another method involves the use of low sodium glass powder which is heated to form low sodium balloons. However, in this method, a finely divided glass frit is needed in order to form good balloon spheres. The preparation of such a frit is time consuming and expensive.
Methods using organic precursor gels have been proposed, however such methods often are expensive, difficult to control, or otherwise ineffective. Accordingly, there remains a need for a simple, economical process for reliably producing low sodium hollow glass microspheres.